Neutron-rich nickel-78 is doubly magic
Two independent experiments have verified that the radioactive nucleus nickel-78 is “doubly magic”, as predicted by the nuclear shell model. The nucleus contains 28 protons and 50 neutrons, which makes it an ideal candidate to test if the shell model applies to nuclei with relatively large numbers of neutrons.
The idea that there are nuclei with “magic numbers” of protons or neutrons (2, 8, 20, 28, 50, 82 and 126) that are more stable than others was first pointed out by Maria Goeppert-Mayer in 1949. It led to the development of the nuclear shell model, for which she shared the 1963 Nobel Prize for Physics.
The idea behind the model is that protons and neutrons fill nuclear orbitals just as electrons fill orbitals in atoms. When a nucleus has a magic number of neutrons, for example, its outer shell of orbitals is completely filled with neutrons. As the energy gaps between shells are relatively big, moving a neutron from a full shell into the next empty shell is difficult, which makes the nucleus relatively stable.
Nickel-78 should be doubly magic because it has full proton and neutron outer shells. Being different particles, protons and neutrons occupy different orbitals. They can, however, feel each other’s presence via the strong force, which perturbs the orbitals so much that the shell model breaks down and new magic numbers emerge. This is known to occur in some nuclei that have high ratios of neutrons to protons.
Now, international teams working independently at the ISOLDE facility at CERN in Switzerland and at the Radioactive Ion Beam Factory (RIBF) at the RIKEN lab in Tokyo have gained important insights into nickel-78 by studying the copper-79 nucleus, which has an extra proton. The RIKEN researchers carried out spectroscopic measurements of copper-79 nuclei in an excited energy state, which revealed that it is best described as having one proton in the next shell above the closed 28-proton shell – thus confirming the shell model. The ISOLDE team took a very different approach and instead made precise measurements of the masses of the copper isotopes copper-75 to copper-79. This allowed them to conclude that copper-79 is best described as a doubly-magic nickel-78 nucleus with one proton in the next shell (Phys. Rev. Lett. 119 192501 and 119 192502).